BIOG 1105-1106 | Cornell Introductory Biology, Individualized Instruction

Thus far we have considered microevolution, the gradual change of a population through time and the processes of speciation. Now we turn to macroevolution, the changes in the kinds of species over time. It involves large-scale evolutionary changes above the level of species, and includes the processes of adaptive radiation, the development of evolutionary trends, the origin of innovative features, and the effect of extinctions.

Biologists are in agreement that the enormous diversity of life on earth today evolved from simple organisms that came into existence over three billion years ago. Clearly, divergent evolution - the evolutionary splitting of species into many separate descendant species - has been exceedingly frequent. Some ancestral species undergo slow, gradual change for millions of years, only occasionally giving rise to other species. Other species undergo bursts of evolutionary activity, producing many separate descendent species that display diverse ways of life even though they are in the same geographical area. The latter process is known as adaptive radiation. Adaptive radiations are common in periods of environmental change or where new ecological opportunities exist. They often occur when new areas open up for colonization, such as isolated island chains, or when some organisms evolve novel structural adaptations allowing them to exploit a new way of life (e.g., insect wings or amphibian legs). Adaptive radiation within the insects, for example, has produced more than 750,000 different living species.

Darwin's finches provide an excellent example of adaptive radiation-an ancestral species introduced to a new environment giving rise to many new species, each adapted for a particular habitat and way of life. The 14 species of Darwin's finches are found nowhere in the world other than the Galapagos Islands (except one species that also inhabits Cocos Island, 700 kilometers to the northeast). They are believed to have evolved on the Galapagos Islands from some unknown finch ancestor that colonized the islands from the South American mainland. Much of the finches' great speciation is due to their having colonized more than 15 separate islands. The finches will not readily fly across wide stretches of water, and they show a strong tendency to remain near their home area. Hence a population on any one of the islands is effectively isolated from the populations on the other islands. It is thought that the initial Galapagos colony was established on one of the islands where the colonizers, perhaps blown by high winds, chanced to land. Later, individuals from this colony wandered or were blown to other islands and founded new colonies. The allelic frequencies in the new colonies must have differed from those in the original colonies from the moment they started, resulting in the founder effect. In time, the colonies on the different islands diverged even more, according to geographic speciation (different mutations, different selection pressures, and, in small populations as some of these must have been, genetic drift). Whenever more than one species occurred together, character displacement could take place, leading to more divergence. (Character displacement is the tendency of closely related species occurring together to diverge rapidly, which reduces the chance of hybridization and/or minimizes competition between them). This is precisely what we find in Darwin's finches - in this case the evolution of different feeding and nesting habits.

Galápagos finches. The Galápagos Islands have a total of 14 species of closely related finchesm some found only on a single island. The most striking difference among species is their beaks, which are adapted for specific diets. (From Campbell, p. 433, Fig. 22.6)

There are two main groups of finches: those that live primarily on the ground, and those that live in the trees, and there is considerable specialization with respect to diet within each group. Correlated with the differences in diet among the species are major differences in the size and shape of their beaks: some large and thick for cracking heavily walled seeds, others smaller and adapted for eating buds or fruits, and still others for eating small soft insects. These characteristics of the beak are apparently the principal means by which the birds recognize other members of their own species and vary their diet.

The process of island-hopping followed by divergence could continue indefinitely and produce many additional species, a process that undoubtedly led to the information of the 14 species of Darwin's finches.

Adaptive radiation is one example of divergent evolution. Divergent evolution is the process of two or more related species becoming more and more dissimilar. The red fox and the kit fox provide and example of two species that have undergone divergent evolution. The red fox lives in mixed farmlands and forests, where its red color helps it blend in with surrounding trees. The kit fox lives on the plains and in the deserts, where its sandy color helps conceal it from prey and predators. The ears of the kit fox are larger than those of the red fox. The kit fox's large ears are an adaptation to its desert environment. The enlarged surface area of its ears helps the fox get rid of excess body heat. Similarities in structure indicate that the red fox and the kit fox had a common ancestor. As they adapted to different environments, the appearance of the two species diverged.s.